Conventionally, aluminum substrates have been widely used as substrates for magnetic disks being one of magnetic recording media for use in hard disk drives (HDDs) and the like. However, following the reduction in size and thickness and the increase in recording density of magnetic disks, glass substrates excellent in substrate surface flatness and substrate strength as compared with the aluminum substrates have been gradually replacing them.
Normally, a glass magnetic disk substrate is manufactured through the following processes.
(1) First Lapping Process (process for making uniform the thickness of a glass base plate)
(2) Shaping Process (process for shaping the glass base plate into an annular glass substrate and chamfering end faces)
(3) Second Lapping Process (process for removing fine irregularities formed on main surfaces in the shaping process)
(4) Surface Polishing Process (process for polishing the main surfaces of the glass substrate; there are first and second polishing processes if necessary)
(5) Chemical Strengthening Process (process for strengthening the strength of the glass substrate)
In the above-mentioned manufacturing processes of the magnetic disk glass substrate, the chemical strengthening process is carried out for increasing the strength of the glass substrate, if necessary. For example, when an aluminosilicate glass is used as the glass substrate, Li+ ions or the like present on the glass surfaces are ion-exchanged with larger ions (e.g. Na+ ions, K+ ions, or the like) by immersing the glass substrate in a predetermined solution. Compressive stress is generated on the ion-exchanged glass surfaces so that the strength of the glass substrate can be increased. Normally, an area (compressive stress layer) where the compressive stress is generated by the chemical strengthening treatment is at most about 100 μm from the glass substrate surfaces.
Hitherto, since higher flying stability of recording/reproducing heads in HDDs has been required, the technique has been developed aiming at more flattening surfaces of HDD glass substrates. However, if only the flatness is simply pursued, there has been a problem that when a glass substrate after processing is slightly saddle-shaped, a flying recording/reproducing head swings up and down to follow it so that the flying of the head becomes unstable. Herein, the saddle shape means a shape like a saddle for use in horse riding and represents a shape that is bent in one direction along a certain diameter as an axis and is further bent in the opposite direction along a diameter perpendicular to the certain diameter, as an axis.
Even if no chemical strengthening treatment is carried out, variation in stress distribution may occur due to the flatness of surface plates used in processing a glass substrate so that the glass substrate may have a saddle shape. Further, there has been a problem that even if a flat glass substrate is obtained without carrying out chemical strengthening treatment, uneven forces may be applied to the inner diameter of the substrate during clamping and, as a result, surfaces of the substrate after incorporation into an HDD may be bent into a saddle shape.
Conventionally, such abnormality in shape did not affect the characteristics of an HDD and thus has not particularly been a problem. However, following the reduction in flying height of a head for achieving higher HDD recording density and the increase in rotational speed for achieving shorter read/write time in recent years, there has been a possibility of affecting the characteristics of an HDD.
With respect to such waviness of the surfaces of the magnetic disk glass substrate, there has been proposed a method of solving it by polishing the substrate surfaces under specific conditions (e.g. Patent Document 1).